This invention relates to an improvement in the process of making a cerium-activated yttrium silicate phosphor of the fast decay type.
Fast decay phosphors have important roles as phosphors for flying spot scanner tubes used in EVR (Electric Video Recording) and index color tubes used in information processing. In a flying spot scanner tube, unlike the television picture tube, a spot on the luminous screen is not modulated but is of constant luminosity and only scans at very high speed. The light from such a flying spot is converged through an optical system and scans a picture or pattern on a film or sheet; the light passing through the film or reflected from the sheet is transformed into a time-sequential electric signal.
For the light source of such a flying spot, the fast decaying characteristic is an important factor. Also, in certain applications, the spectrum distribution is an important characteristic. For instance, for obtaining an electric signal of a color film, the flying spot source should have a spectrum distribution to cover the three principal colors in order to generate three electric signals corresponding to the principal colors. Further, when scanning a monochromic (e.g. black and white) picture or pattern, a high luminosity is preferable for monitoring with the naked eye.
It has been known that cerium-activated phosphors, for instance Ca.sub.2 MgSiO.sub.7 :Ce, Ca.sub.2 Al.sub.2 SiO.sub.7 :Ce, YPO.sub.4 :Ce, Y.sub.3 (Al,Ga).sub.5 O.sub.12 :Ce, Y.sub.2 SiO.sub.5 :Ce and Y.sub.2 SiO.sub.7 :Ce, have a fast decaying emission at the electron beam excitation. However, with the exception of Y.sub.3 (Al,Ga).sub.5 O.sub.12 :Ce, which emits yellow light, these phosphors emit only near-ultraviolet light or near-ultraviolet blue light and none are satisfactory as a phosphor for a color flying-spot tube, nor are they satisfactory even as a blue-emitting phosphor component of a white light flying spot tube.
In 1969, A. H. Gomes & A. Brill reported on Ce-activated yttrium silicates Y.sub.2 SiO.sub.5 :Ce and Y.sub.2 Si.sub.2 O.sub.7 :Ce. The Y.sub.2 Si.sub.2 O.sub.7 :Ce phosphor has its peak emission at around 380 nm and has the highest energy conversing efficiency among the above-mentioned Ce-activated phosphors. The Y.sub.2 SiO.sub.5 :Ce phosphor has the second highest energy-conversion efficiency and its peak in the spectrum curve is at around 400 nm.
In the past, these Ce-activated yttrium silicate phosphors have been made by firing a mixture of oxides of the necessary component materials in the required mol quantities in the air or in a reducing atmosphere. During the firing of these phosphors, YF.sub.3 (yttrium fluoride) was conventionally used as a mineralizer or flux for accelerating the reaction but it was found difficult to obtain a single composition of .beta.-Y.sub.2 Si.sub.2 O.sub.7 :Ce, that is, one consisting of only .beta.-phase composition excluding the components of the other phases.